“Leading a sedentary lifestyle may make us genetically old before our time,” according to the BBC news website today. It reports on a UK study of twins that looked at the association between physical activity and the length of telomeres, pieces of DNA that “cap the ends of chromosomes… and protect them from damage”.

The report explains that as someone gets older, their telomeres get shorter, leaving their cells more exposed to damage and death. The study found that twins who were physically active had longer telomeres than those who were inactive. The researchers concluded that their results highlight the importance of following advice to keep physically active.

This news story comes from a study of 2,401 twins in the UK. Although it does show an association between telomere length and exercise, it does not prove that exercise affected telomere length as the study only examined individuals once. Scientists are still some way from being able to explain the effect that physical activity has on individual cells. This study contributes to our knowledge.

Even without telomere research, there is good evidence that keeping physically active will promote a longer, healthier life.

Where did the story come from?

Dr Lynn Cherkas and colleagues from Kings College London and the New Jersey Medical School, carried out the research. The study was funded by the Welcome Trust, the National Institutes of Health and The Healthcare Foundation of New Jersey. It was published in the peer-reviewed medical journal Archives of Internal Medicine .

What kind of scientific study was this?

This cross-sectional study looked at how physical activity affects the length of the telomeres, the DNA that is situated at the ends of chromosomes. There is a suggestion that telomere length is an indicator of the ageing process, with telomeres getting shorter as someone ages.

Using a UK register of adult twins who had submitted postal questionnaires, the researchers selected 2,401 pairs of white twins, aged 18 to 81 and consisting of 2,152 women and 249 men. Of these, 915 pairs were non-identical, 167 pairs were identical, and 237 people enrolled without their sibling.

The twins filled out questionnaires about themselves, their health, and their lifestyles. This included questions about their physical activity during work and leisure time over the past 12 months and before, their age, socioeconomic status, and whether they smoked. During a clinical examination the twins were weighed and measured to calculate their body mass index (BMI), and blood samples were taken and their white blood cells extracted.

The researchers extracted the DNA from the cells and examined the average length of each person’s telomeres. The telomere lengths for people with different levels of physical activity were then compared. The researchers took into account factors that might affect the results, including age, smoking, physical activity at work, BMI, socioeconomic status and gender. They also looked at the differences in telomere length within 67 pairs of twins who were brought up together but did different amounts of physical activity, to see if the more active twin had a different telomere length to the less active twin.

What were the results of the study?

The researchers found that, in an average week, the least active had performed 16 minutes of physical activity in their leisure time, while the most active had performed 199 minutes (just under three hours and 20 minutes) in the past 12 months.

The more active someone was in their leisure time, the longer their white blood cell telomeres were, even when age, smoking and activity at work were taken into account.

The most active people had telomeres that were on average 200 nucleotides (the building blocks of DNA) longer than the least active people. The researchers estimated that this was equivalent to having the telomere length of a person up to 10 years younger. When pairs of twins with different activity levels were looked at, it was found that the more active twin’s telomeres were on average 88 nucleotides longer than the less active twin.

What interpretations did the researchers draw from these results?

The researchers concluded that being inactive “has an effect” on telomere length in white blood cells, and “may accelerate the ageing process”.

What does the NHS Knowledge Service make of this study?

This study aimed to link the known benefits of exercise with a biological indicator of ageing: telomere length. It is a good quality study, but there are a few limitations to note:

The study only looked at white blood cell telomeres, other cells of the body might give different results.

Most of the people in the study were women, and there were too few men to detect an association between telomere length and physical activity in men alone. Therefore, it is not possible to be certain that these results would be the same in men.

This type of study (a cross-sectional study) does not assess the sequence of events, and therefore, cannot provide evidence that one event (in this case physical activity) causes another (telomere shortening). The authors note that a long-term prospective study is needed to assess this relationship further.

This study did not assess whether the people with shorter telomeres had more signs of aging than those with longer telomeres of a similar age.

The main limitation when interpreting the results of this study is that a person’s level of activity will be affected by a large number of factors; for example, if they are in poor health they may exercise less. It was found here that the more active people had lower BMIs, were less likely to smoke, and were less likely to be manual workers. However, their likelihood of being affected by chronic disease was no different from less active people. It could be these or other factors that are causing the telomeres to be shorter, and not the lack of physical activity. The researchers did try to take these factors into account, but it is difficult to know if the adjustments have fully removed the effect of the known factors, and there may be other factors that have not been taken into account.

At present, it is unknown if increased physical activity will delay or prevent telomere shortening, and if it can be prevented, whether it would slow ageing or improve health.

The benefits of a physically active life over a sedentary one are already well known, including the reduction in risk of many diseases that can shorten lifespan. Until more is known about the effects or otherwise of telomere length, a healthy lifestyle with physical activity should be the goal anyway.

Sir Muir Gray adds...

Do not rely on the length of your telomeres to keep you fit: rely instead on the length of your walks. In each decade of your life you need to take activity more often, not less. Many of the effects attributed to aging are due to loss of fitness and not to the genes.